Conventionally, the input threshold level of a device under test is given by the input signal level when a signal having a level varying in steps is input to a device under test and the output signal of the device under test varies, or it is calculated from the delay time from the signal input, when a ramp signal is input to the device under test, to the change in the output signal of the device under test and the gradient of the slope part of the ramp signal (i.e., see JP Unexamined Patent Application No. H3-216,565 (p. 2, FIG. 2, FIG. 5)). The level denotes the magnitude in the amplitude direction.
A method for inputting a signal having a level varying in steps to a device under test is a static test and cannot measure a dynamic threshold level in the actual operating environment of the device under test. In addition, the above-mentioned method for inputting a ramp signal to a device under test requires a time measurement device synchronized to the input ramp signal in order to know the delay time. Therefore, an object of the present invention is to provide a method or an apparatus for measuring the input threshold level of a device under test without synchronizing to the signal input to the device under test.